Combination of casting process and alloy composition

ABSTRACT

A process for casting a magnesium alloy consisting of 10.00-13.00% by weight of aluminium, 0.00-10.00% by weight of zinc, and 5.00-13.00% by weight of aluminium, 10.00-22.00% by weight of zinc, also containing 0.10-0.5% by weight of manganese, and the balance being magnesium and unavoidable impurities, the total impurity level being below 0.0% by weight, wherein the alloy is cast in a die in which the temperature is controlled in the range of 150-340° C., the die is filled in a time which expressed in milliseconds is equal to the product of a number between 2 and 300 multiplied by the average part thickness expressed in millimetre, the static metal pressures being maintained during casting between 20-70 MPa and may subsequently be intensified up to 180 MPa.

The present invention relates to a process for casting a magnesium alloyconsisting of aluminium, zinc and manganese, and the balance beingmagnesium and unavoidable impurities, the total impurity level beingbelow given % by weight.

Magnesium-based alloys are widely used as cast parts in automotiveindustries, and with increasing importance in 3C components (3C:computers, cameras and communications). Magnesium-based alloy cast partscan be produced by conventional casting methods, which includedie-casting, sand casting, permanent and semi-permanent mould casting,plaster-mould casting and investment casting.

Mg-based alloys demonstrate a number of particularly advantageousproperties that have prompted an increased demand for magnesium-basedalloy cast parts in the automotive industry. These properties includelow density, high strength-to-weight ratio, good castability, easymachinability and good damping characteristics. Most common magnesiumdie-casting alloys are such as Mg—Al-alloys or Mg—Al—Zn-alloys with<0.5% Mn, mainly Mg-9% Al-1% Zn (designated AZ91), Mg-6% Al (AM60) andMg-5% Al (AM50).

WO 2006/000022 A1 describes a magnesium-based alloy containing zinc,aluminium, calcium and/or beryllium or optionally manganese by which isprovided an attempt to improve the surface finish of cast magnesiumcomponents. The WO reference is, however, not particularly concernedwith the castability of the alloy.

With the present invention is provided to provide relatively low costmagnesium-based alloy with improved surface finish and improvedcastability.

The invention is characterized by an alloy containing

-   -   10.00-13.00% by weight of aluminium,    -   0.00-10.00% by weight of zinc, and    -   5.00-13.00% by weight of aluminium,    -   10.00-22.00% by weight of zinc,        also containing    -   0.10-0.5% by weight of manganese,        and the balance being magnesium and unavoidable impurities, the        total impurity level being below 0.1% by weight, whereby    -   the alloy is cast in a die the temperature of which is        controlled in the range of 150-340° C.,    -   the die is filled in a time which expressed in milliseconds is        equal to the product of a number between 2 and 300 multiplied by        the average part thickness expressed in millimeter,    -   the static metal pressures being maintained during casting        between 20-70 MPa and may be subsequently intensified up to 180        MPa, as defined in the attached independent claim 1.

Dependent claims 2-11 define preferred embodiments of the invention.

By using the combination of a specified Mg—Al—Zn alloy with the specialcasting process as defined above, products may be made having excellentsurface finish, reasonable ductility and acceptable mechanicalproperties as well as corrosion properties.

Preferably the aluminium content is between 5.00 and 13.00% by weight.If less than 10.00% Al is present the Zn content is restricted to10.00-22.00% by weight. Lower Zn contents give poorer combination ofcastablity and surface finish.

If more than 10.00% Al is present, the range of Zn can be extended to0.00-22.00% still giving satisfactory castability and surface finish.

For applications requiring a minimum of ductility the composition of thealloy is selected in such a way that the aluminium content is between10.00 and 12.00% by weight and the Zn-content is between 0.00 and 4.00%by weight. Alloys with equivalent castability and surface finish can beprepared if the composition of the alloy is such that the aluminiumcontent is between 6.00 and 12.00% by weight and the Zn-content isbetween 10.00 and 22.00% by weight. These alloys offer the advantages oflower casting temperature.

The present invention will be further described in the following bymeans of examples and with reference to the attached drawings where:

FIGS. 1A, B each shows schematically cold chamber and hot chamber diecasting machines, respectively,

FIG. 2 is a diagram showing the relationship between the solidificationrate and the microstructure (grain size and secondary dendrite armspacing) of cast Mg alloys,

FIG. 3 is a diagram showing the grain size vs. ductility of Mg alloys,

FIG. 4 is a diagram showing the grain size vs. tensile yield strength ofMg alloys,

FIG. 5 shows a chart from a prior art reference, G. S Foerster; “Newdevelopments in magnesium die casting”, IMA proceedings 1976 p. 35-39,who split the composition range into a castable—, a brittle—and a hotcracking region,

FIG. 6 shows the Mg-rich corner of the Mg—Al—Zn phase diagram with linesof constant liquidus temperature,

FIG. 7 shows a diagram with the fraction solid (expressed in % byweight) on the horizontal axis versus the temperature (OC) on thevertical axis for three different Mg alloys,

FIGS. 8-10 show three different Mg alloy components being cast withthree different dies,

FIG. 11 is a diagram showing casting defects, average number of cracksand defect ribs on the box die, FIG. 8, plotted as lines of equal numberof defects in a diagram, where the Zn content is plotted along thex-axis and the Al content along the y-axis,

FIG. 12 is a diagram showing surface finish represented as a rating from1 to 5 on the box die, FIG. 8, plotted as lines of equal rating in adiagram, where the Zn content is plotted along the x-axis and the Alcontent along the y-axis,

FIG. 13 is a diagram showing where the z-axis is representing thetensile strength expressed in MPa, while the x and y-axes arerepresenting the Al and Zn contents, respectively, and where theductility is represented as lines of equal % elongation in the samediagram,

FIG. 14 is a diagram showing corrosion rates in terms of weight lossbeing represented as lines of equal corrosion rates (mg/cm²/day), wherethe Zn content is plotted along the y-axis and the Al content along thex-axis.

In FIGS. 1A and 1B there are schematically shown cold chamber and hotchamber die castings machines respectively, each machine has a die 10,20 provided with a hydraulic clamping system 11, 21, respectively.

Molten metal is introduced into the die by means of a shot cylinder 12,22 provided with a piston 13, 23, respectively. In the cold chambersystem as is shown in FIG. 1A, an auxiliary system for metering of themetal to the horizontal shot cylinder is required. The hot chambermachine, however, shown in FIG. 1 B, uses a vertical piston system 12,23 directly in the molten alloy.

To obtain the excellent performance of the Mg—Al—Zn alloys, it ismandatory that the alloys are cast under extremely rapid coolingconditions. This is the case for the high pressure die casting process.The steel die 10, 20 is equipped with an oil (or water) cooling systemcontrolling the die temperature in the range of 200-300° C. Aprerequisite for good quality is a short die filling time to avoidsolidification of metal during filling. A die filling time in the orderof 10⁻² s× average part thickness (mm) is recommended. This is obtainedby forcing the alloy through a gate with high speeds typically in therange 30-300 m/s. Plunger velocities up to 10 m/s with sufficientlylarge diameters are being used to obtain the desired volume flows in theshot cylinder for the short filling times needed. It is common to usestatic metal pressures 20-70 MPa and subsequent pressure intensificationup to 180 MPa may be used, especially with thicker walled castings. Withthis casting method the resulting cooling rate of the component istypically in the range of 10-1000° C./s depending on the thickness ofthe component being cast.

In FIG. 2 there is shown the relationship between the solidificationrange and the microstructure of a cast alloy. On the horizontal axisthere is shown the solidification rate expressed as ° C./s and on theleft hand vertical scale the secondary dendrite arm spacing expressed inμm is shown, whereas on the right hand vertical scale the grain diameterexpressed in μm is shown. Line 30 indicates the grain size obtained,whereas line 31 is the obtained value for the secondary dendrite armspacing.

With die casting grain refining is obtained by the cooling rate. Asmentioned above cooling rates in the range of 10-1000° C./s are normallyachieved. This typically results in grain sizes in the range of 5-100μm.

It is well known that fine grain size is beneficial for the ductility ofan alloy. This relationship is illustrated in the annexed FIG. 3, inwhich the relationship between grain size and relative elongation hasbeen shown. On the horizontal axis the average grain size has beenrepresented expressed in μm, whereas the vertical axis gives therelative elongation expressed in %. In the graph there are shown twodifferent compositions, first pure Mg, line 35 and an Mg-alloydesignated AZ91 (Mg-9% Al, 1% Zn), line 36.

It is also well known that fine grain size is beneficial for the tensileyield strength of an alloy. This relationship (Hall-Petch) is shown inthe annexed FIG. 4. In the horizontal axis there is represented thegrain diameter, expressed as d^((−0.5)), in which d has been expressedin μm, and in the vertical axis there is shown the tensile yieldstrength expressed in MPa.

It is therefore evident that the fine grain size provided by the veryhigh cooling rates facilitated by the die casting process is a necessityfor obtaining tensile strength and ductility.

The castability term describes the ability of an alloy to be cast into afinal product with required functionalities and properties. It generallycontains 3 categories; (1) the ability to form a part with all desiredgeometry features and dimensions, (2) the ability to produce a densepart with desired properties, and (3) the effects on die cast tooling,foundry equipment and die casting process efficiency.

In the 3C industry extremely thin-walled components for e.g. lap-top andcell phone housings, often less than 0.5 mm, are cast. This puts strongrequirements on the ability of the alloy to fill the mould and at thesame time provide a smooth and shiny surface. AZ91 is the most commonalloy for these applications, mainly due to the better castabilitycompared to AM50 and AM60. However, the surfaces of thin walledcomponents of AZ91 are often not satisfactory. Usually, a conversioncoating is applied to these components. With a less shiny surfacesometimes including areas with segregation of elements, multiple layersof coating has to be used. Generally, the better surface quality, theless coating is needed.

Mg—Al—Zn alloys with 0-10 wt % Al and 0-35 wt % Zn were examined in the1970's (G. S Foerster; “New developments in magnesium die casting”, IMAproceedings 1976 pp. 35-39). The chart shown in FIG. 5, from Foerster'spaper, split the composition range into a castable—, a brittle—and a hotcracking region. The alloys described in Australian patent WO2006/000022 A1 that provide an attempt to improve the surface finish,are mainly inside the castable region of FIG. 5. The alloy compositionranges of the present invention are mainly outside the compositionranges described in the prior art (FIG. 5) and completely outside thosedescribed in patent WO 2006/000022 A1. During the tests that will beexplained later it became evident that the alloys of the presentinvention represent considerable improvements over the earlier describedalloys in terms of die filling, die sticking and hot cracking. These areall crucial features in die casting of complex thin-walled components.

The Mg—Al—Zn alloys with the Al and Zn content as specified in thepresent invention will start to solidify around 600° C., depending onthe Al and Zn content. This is indicated in FIG. 6 where lines ofconstant liquidus temperature in the Mg-corner of the Mg-AI—Zn phasediagram are shown. As a result, the casting temperature, typically 70°C. above the liquidus, can be significantly lower than for theconventional AM50, AM60 and AZ91 alloys. Due to the fact that theeutectic Mg₁₇Al₁₂ phase melts at around 420° C., the conventional Mg—Alalloys like AM50, AM60 and AZ91 will have a solidification range ofnearly 200° C. as shown in the annexed FIG. 7 which shows the fractionsolid (expressed in % by weight) on the horizontal axis versus thetemperature (° C.) on the vertical axis for three different alloys.Specifically, AZ91 starts to solidify at 600° C. and is completelysolidified at 420° C. Increasing the Al content to 14% as in alloyAZ141, the start of solidification occurs at around 570° C. whilesolidification is complete at 420° C. Due to the significant presence ofZn the alloy AZ85 solidifies in the range 590-350° C. Since Zn in theMg—Al—Zn alloy modifies the eutectic Mg₁₇Al₁₂ phase, the alloy willsolidify completely at temperatures significantly lower than 420° C. asis the case for the conventional alloys AM50, AM60 and AZ91.

In general, increasing aluminium content in Mg—Al die casting alloysimproves the die castability. This is due to the fact that Mg—Al alloyshave a wide solidification range, which makes them inherently difficultto cast unless a sufficiently large amount of eutectic is present at theend of solidification. This can explain the good castability of AZ91Dconsistent with the cooling curves shown in FIG. 7. With the largeamount of Zn in addition to Al in the present alloys there is an evenlarger amount of (modified) eutectic present at the end ofsolidification, explaining the improved castability of the inventedMg—Al—Zn alloys.

Magnesium alloys tend to ignite and oxidize (burn) in the molten stateunless protected by cover gases such as SF₆ and dry air with or withoutCO₂, or SO₂ and dry air. The oxidation aggravates with increasingtemperature. Usually, small amounts of beryllium (10-15 ppm by weight)are also added to reduce the oxidation. Beryllium is known to form toxicsubstances and should be used with care. Especially the treatment ofdross and sludge from the cleaning of crucibles requires considerablesafety precautions due to an enrichment of Be-compounds in dross/sludge.One advantage of the present invention is that the alloy can be cast attemperatures significantly lower than for conventional alloys, therebyreducing the need for cover gases. For the same reason, berylliumadditions can be kept at a minimum.

The lower casting temperatures compared to conventional alloys alsooffer significant advantages as the lifetime of the metering system, theshot cylinder and the die will all be improved. With hot chamber diecasting in particular, the lifetime of the gooseneck will besignificantly extended. The alloys with lower casting temperature alsohave a potential for reducing the cycle time, thereby improving theproductivity of the die casting operation.

EXAMPLE 1

In order to evaluate the influence of the alloying elements and a numberof Mg-alloys have been prepared and cast in three different dies:

-   -   The box die with ribs, FIG. 8    -   The plate/bar die, FIG. 9    -   The three plate die, FIG. 10

The alloy compositions and the casting temperatures are shown in Table 1below.

TABLE 1 Casting Al (wt %) Zn (wt %) Temp (C.) AM20 2 0 710 AZ21 2 1 710AZ22 2 2 705 AZ2-3.5 2 3.5 700 AM40 4 0 700 AZ41 4 1 695 AZ42 4 2 695AZ4-3.5 4 3.5 690 AZ45 4 5 680 AZ4-14 4 14 650 AZ4-18 4 18 630 AM60 6 0680 AZ61 6 1 680 AZ62 6 2 680 AZ63 6 3 680 AZ6-3.5 6 3.5 680 AZ65 6 5670 AZ66 6 6 670 AZ6-12 6 12 640 AZ6-18 6 18 610 AZ71 7 1 680 AZ72 7 2680 AM80 8 0 680 AZ81 8 1 680 AZ82 8 2 670 AZ8-3.5 8 3.5 670 AZ85 8 5670 AM90 9 0 670 AZ91 9 1 670 AZ96 9 6 650 AZ99 9 9 640 AZ9-12 9 12 620AZ9-18 9 18 585 AZ9-22 9 22 560 AM100 10 0 660 AZ10-1 10 1 660 AZ10-2 102 660 AZ10-3.5 10 3.5 650 AZ10-5 10 5 650 AM120 12 0 650 AZ12-1 12 1 650AZ12-2 12 2 640 AZ12-3.5 12 3.5 640 AZ12-5 12 5 630 AZ12-6 12 6 630AZ12-12 12 12 590 AZ12-18 12 18 550 AM140 14 0 640 AZ14-1 14 1 630AZ14-2 14 2 630 AZ14-3.5 14 3.5 620 AZ14-5 14 5 610

Details of the casting parameters are given in Table 2 below.

TABLE 2 Calculated Velocity 1 Velocity 2 Braking Fill Time (m/s) (m/s)(m/s) (ms) Die 1 Tensile specimen 0.5 5 3 50 Die 2 Three Plate 0.5 5 2.553 Die 3 Box 0.5 5 3 40

No intensification pressure was used.

The performed tests are the following:

Evaluation of Casting Defects

-   -   Visual inspection was undertaken on 10 arbitrary boxes from each        alloy.    -   Defects were grouped as        -   Defect ribs including incomplete filling and cold shuts        -   Hot tears counted on nodes        -   End cracks

Evaluation of Surface Finish

-   -   Surface finish was inspected visually by several persons        independently, and rated from 1 to 5 (5 best).

Tensile Strength and Ductility

-   -   Test-bars of 6 mm diameter in accordance to ASTM B557M have been        made, and the following test conditions have been used:        -   10 kN Instron test machine        -   Room temperature        -   At least 10 parallels        -   Strain rate            -   1.5 mm/min up to 0.5% strain,            -   10 mm/min above 0.5% strain        -   Testing in accordance with ISO 6892

Corrosion Properties

-   -   The corrosion tests were done according to ASTM B117.

EXAMPLE 2

Casting defects average number of cracks and defect ribs are plotted inFIG. 11 as lines of equal number of defects in a diagram where the Zncontent is plotted along the x-axis and the Al content along the y-axis.It is seen that the lowest numbers of cracks are found in the regionswith low Zn (<3%) and high Zn (≦10%). It is seen that that particularlygood alloys in terms of casting defects are found with Al in the rangeof 8-10% by weight and with Zn<2% by weight; the lower Zn the better.Also, alloys with Al in the range of 7-12% by weight and Zn in the range12-18% by weight exhibit very few casting defects.

EXAMPLE 3

Surface finish represented as a rating from 1 to 5 is plotted in FIG. 12as lines of equal rating in a diagram where the Zn content is plottedalong the x-axis and the Al content along the y-axis. It is seen thatthe best regions in terms of surface finish rating are found with Al>11%by weight and Zn<3% by weight; the lower Zn the better. Also, a regionroughly defined by 8-12% Al by weight and >10% Zn by weight providesalloys with superior surface finish.

EXAMPLE 4

For a number of compositions the strength and elongation have beenmeasured at room temperature. The results are shown in FIG. 13. Here,the z-axis is representing the tensile strength expressed in MPa,whereas the x and y-axes are representing the Al and Zn contents,respectively. The ductility is represented as lines of equal elongationin the same diagram. Generally it is seen that tensile strengthexpressed in MPa, increases with increasing content of alloyingelements. The effect of increasing Al (% by weight) is significantlygreater than the effect of Zn. FIG. 13 also indicates that the ductilityin terms of % elongation decreases with increasing content of alloyingelements. As an example, the line indicating 3% elongation extendsalmost linearly from 12% Al by weight and 0% Zn to 0% Al and 18% Zn byweight.

EXAMPLE 5

For a number of compositions the corrosion properties have been definedin accordance to ASTM B117. In this test a great amount of data has beenincorporated in order to define the influence of the Zn-content versusthe Al-content. The results are shown in FIG. 14.

In this figure corrosion rates in terms of weight loss is represented aslines of equal corrosion rates (mg/cm²/day), in a diagram where the Zncontent is plotted along the y-axis and the Al content along the x-axis.It is seen that for Zn contents lower than approximately 8% by weight,the corrosion rates decrease with increasing Al content and arepractically independent of the Zn content, whereas for Zn contents aboveapproximately 12% by weight the corrosion rates increases slightly withincreasing Zn content and are practically independent of the Al content.The region defined by 8-12% by weight of Zn represents a transition.Specifically, at 0% Zn the corrosion rate decreases from about 0.09mg/cm²/day at 4% Al by weight to approximately 0.03 mg/cm²/day at 9% Alby weight. At constant 9% Al by weight the corrosion rate increases to0.05 mg/cm²/day at 8% Zn by weight and 0.11 mg/cm²/day at 14% Zn byweight.

From these test results it is clear that a process for casting amagnesium alloy has been provided whereby products are obtained with asuperior combination of elevated temperature creep properties, ductilityand corrosion performance.

1. A process for casting a magnesium alloy consisting of 10.00-13.00% byweight of aluminium, 0.00-10.00% by weight of zinc, and 5.00-13.00% byweight of aluminium, 10.00-22.00% by weight of zinc, also containing0.10-0.5% by weight of manganese, and the balance being magnesium andunavoidable impurities, the total impurity level being below 0.1% byweight, wherein the alloy is cast in a die in which the temperature iscontrolled in the range of 150-340° C., the die is filled in a timewhich expressed in milliseconds, is equal to the product of a numberbetween 2 and 300 multiplied by the average part thickness expressed inmillimeter, the static metal pressures being maintained during castingis between 20-70 MPa and may subsequently be intensified up to 180 MPa.2. A process according to claim 1, characterized in that the dietemperature is controlled to a temperature in the range between 160 and300° C., preferably in the range between 200 and 270° C.
 3. A processaccording to claim 1, characterized in that the filling time of the dieexpressed in milliseconds is equal to the product of the average partthickness expressed in millimetre multiplied by a number between 2 and200, preferably between 3 and 50, most preferably between 3 and
 20. 4. Aprocess according to claim 3, characterized in that the static metalpressure during casting is maintained between 30-70 MPa.
 5. A processaccording to claim 4, characterized in that the cooling rate aftercasting is in the range of 10-1000° C./s.
 6. A process according toclaim 5, characterized in that the aluminium content is between 10.00and 13.00% by weight, preferably between 10.00 and 12.00% by weight. 7.A process according to claim 6, characterized in that the Zn content isbetween 0.00 and 10.00% by weight.
 8. A process according to claim 7,characterized in that the aluminium content is between 10.00 and 12.00%by weight and the Zn-content is between 0.00 and 4.00% by weight.
 9. Theprocess according to claim 5, characterized in that the aluminiumcontent is between 5.00 and 13.00% by weight, preferably between 6.00and 12.00% by weight.
 10. The process according to claim 9,characterized in that the Zn content is between 10.00 and 22.00% byweight.
 11. The process according to claim 10, characterized in that thealuminium content is between 6.00 and 12.00% by weight and theZn-content is between 10.00 and 18.00% by weight.